Geochemical prospecting



Jan. 18, 1944. D. s. REARDEN ETAL. 2,339,651-

GEOCHEMICAL PROSPECTING I med Feb.. 29. 1940 Patented Jan. 18, 1944 GEOCHEBIICAL PROSPECTING Donald S. Reardon and Earl G. Brewster, Los

Angeles, and Robert Thomas Sanderson, Sierra Madre, Calif., assignors to Stanolind Oil and Gas Company, Tulsa, Okla., a corporation of Delaware Application v,February 29, `1940, seriaiNo. 321,602

claims. (c1. 23-257) This invention relates to soil gas analysis and more particularly to soil gas analysis for purposes of geochemical prospecting. Still more particularly it relates to methods for the collection of samples of soi1 gas/hydrocarbons and the like for analysis as a step in the art of geochemical prospecting as Well as to apparatus for this purpose.

In recent years increasing attention has been given to theanalysis of soils as an aid to the location of deep-seated oil and gas deposits. The theory commonly held is that hydrocarbon materials migrate upward from deep-seated deposits and are held in or sorbed by' the soils. Thus, by vanalysis of these soils for hydrocarbon materials, and in some instances for related materials such as hydrogen, some indication of the presence of deep-seated petroleum deposits can be had. However, difficulties are encountered in this method of geochemical prospecting due to the fact that the sorptive capacities and retentivities of lsurface soils vary widely so that in the course of a` survey it is diicult to determine Whether anomalies are due entirely to sub-surface conditions or whether they are due in whole or in part to dilerences in the soil samples themselves as to sorptive capacity or retentivity. Furthermore, not only dol soils vary markedly in sorptive capacity due to variations in their chemical compositions and physical natures but the concentration of hydrocarbons and other materials in a given soil may also be dependent on the environmenta1 histrory'of the-soil whichcannot be -known with certainty and which may diierl from survey station, to survey station.

To overcome thesedifficulties one of us has proposed the use of activated charcoal or other of water which must be dealt with is veryI large since the gas in the hole in which the sorbent is suspended is free-to circulate back to the soil, picking up more water without picking up any appreciable incremental amount of hydrocarbons so that the amount of water vapor which must be removed per unit amount of hydrocarbon is.

often very large.

It is an object of our invention to overcome these diinculties and to provide a method of and apparatus for geochemical prospecting which will give more significant and comparable results than the methods and apparatus heretofore used.

It is also an object of our invention to provide a particularly convenient method of sample collection in connection with geochemical prospecting.'

A`further object of our invention is to'lstandardize the collection of samples of volatile soil gas f carbon sample collected. Other and more de- Y tailed objects, advantages and uses of our inventainer in the ground for a predetermined period of time and then removed for analysis of the sorbed hydrocarbons. This is covered by patent application Serial No. 311,812, led December 30,

tion will become apparent as 'the description thereof proceeds.

In brief we accomplish these objects by placing in a cavity in the soil or in communication with a cavity in the soil a sample collection train having an inlet and an outlet with'a provision relatively near the inletend for removing at least one contaminating impurity, notably water, and a provision relatively near the outlet end for collect- 1939, and, as is pointed out therein,4 it-is highly important to protect the hydrocarbon sorbent the soil gases in a hole in the ground in a static manner has marked advantages over prior art technique, it is not without disadvantages since it is dependent upon diiusion and incidental, un-

controlled and unstandardized gas circulation due to thermal and other causes. Another disadvantage of this technique is that the amount ing the hydrocarbon sample. Circulation is so that the amount of gas passing through it is' controllable and s that greater speed of sample collection can b emaintained, also so that gases which have once passed through the sample col-l ..not pass lecting train are'discarded and do through it more than once.

It will be apparent that of this can be vac-v complished in a considerable variety of ways but n it will be described with particular reference to certain preferred embodiments shown in the ac-- companying drawing which formsa part o! this in water or oil wells can', of course,.be use speciiication and are to be read in conjunction therewith and in which like reference numerals refer to like or corresponding parts.

In the drawing:

Figure 1 is a sectionthrough a -portion of the `earths crust showing one embodiment of our invention; s Figure 2 is a detail, in sectional elevation, coreliminated. i After such pretreatment followed by responding to theupper end -of the apparatus shown in Figure 1;

shown is installed by iirst digging a hole II which .maybe about a foot deep and a foot or two on a side. Then a. hole I2'is dug or drilled which may, for instance, be 3 inches in diameter and from 3 to 25 feet deep, for instance 6 or 8 feet deep. In the bottom of hole II is placed a board I3 having an aperture I4 through which is inserted a sample collecting train. Looking at this sample collecting train from the bottom may be merely apiece of pipe with holes I6 cut in it to admit the soil gas `and lined with wire screen or gauze to keep out soil particles. More elaboratescreens or strainers of the typesused but are not usually required. This gas entrance section I is connected by the lowermost of couplings II with the lowermost oi a series of nipples or pipe sections I8'which are lled with sorbents and which are interconnected by the remainder degassing the specific sorptive capacity of the activated charcoal for hydrocarbon gases is higher than it would be if the pretreatment were omitted. ,o

While activated charcoal and similar. carbonaceous sorbents, preferably pretreated and degassed as described, are the preferred hydrocarbon sorbents, other solid sorbents can be used, for instance silicagel. The lower sections of the sample collecting train are utilized 'to remove contaminating substances which interfere with the hydrocarbon sorption. Most important in this connection is Water vapor. As illustrating the importance of the problems introduced by the presence of water vapor in soil gas, an activated charcoal which sorbed 1.15% of its weight of water when exposed to air containing water vapor having a partial pressure of 5.15 millimeters of mercury sorbed 39.33% of water when the partial pressure was 23.76 millimeters of mercury. These diilerences in water sorption greatly aiect the lwdrocarbon sorption and thus lead to non-comparable results. On the other hand, this same activated charcoal would sorb only 0.15% of water from air dried vit consists of a gas entrance section I5 which of couplings II as shown. It will be understood that any desired number of these nipples or pipe sections can be used: They may, for example, be made of ordinary l; inch pipes with each nipple about one foot long. X

`for hydrocarbons, hydrogen, etc., can be used.

Commercial activated chacoal is not generally satisfactory as suchv since it is usually contaminated with non-constant amounts of hydrocarbons or gases which interferwith the hydro.- carbon sorption and especially with 'the subse.- quent analysis. Commercial'activated charcoal must therefore be thoroughly egassed by some such method as prolonged heat yg at a high temperature vacuo before it is t for use as a standard sorbent in accordance with our invention. By way of example, degassing an be carried out by heating the activated grcoal to 600 C. for 10 hours at a out 10-5 millimeters of mercury al ough less vigorous treatment is often entirely sufficient. "It `s advantageous to heat the charcoal, afte brief\pre liminary evacuation, in an jatmo here' of natural (hydrocarbon) 'gas or other gaseous hydrocarbon or hydrocarbons, before subjecting y,the

ESSuIe 0f ff'charcoal. to the Arigorous degassing mentioned labove.

over anhydrous or relatively anhydrous calcium chloride and lwhen all samples in a given survey are thus dried before sorption of the hydrocarbons the eiect of water 'becomes completely negligible.

Instead of calcium chloride other desiccants. for instance magnesium perchlorate, can be used.

Accordingly, the lower section or sections of the sample collecting train are filled with a desiccant, for instance the lower end of the lowermost nipple or pipesection I8 can be lled with commercial calcium chloride and the upper end of the same pipe section with anhydrous calcium chloride, the use of the commercial material to remove the bulk of the water being a matter of economy. This calcium chloride section-of the train 'can suitably be somewhat longer than the other sections.

It is also desirable tov protect the hydrocarbon sorbent in the uppermost section I8 from basic gases and vapors, particularly ammonia, and from acidic gases and vapors, notably carbon dioxide and also sulfur dioxide. Ammonia and other alkaline or basic gases can be removed by the use of sodium bisulfate and various other acidic sorbents can be used as sorbents for ammonia and other alkaline or basic gases, and a material of this type is accordingly placed in the second from the bottom nipple I8. Similarlyv Ascarite (a commercial material containing potassium hydroxide and asbestos) or fused lpotassium or sodium hydroxide or other basic or alkaline sorbent can be used as an acidic gas sorbent. The section of the sample collecting train above the calcium chloride section can thus be lled with sodium bisulfate and the next higher section with Ascarite.

Since the reaction of a basic material such as Ascarite with acidic gases-"produces water, these must be removed by another section filled with i depending upon the elciency of the selective .sorbents used and on the amounts of the various substances encountered in the particular type ot Y soil gas which is met with ln any particular Survey.

use. The other pipe sections areV lled with the selective sorbents indicated and may similarly'be stoppered while being transported to the point of use. The Stoppers are then removed and they are joined together in the proper order overv the gas entrance section I by means ofthe couplings I1. The uppermost sorbent section I8 is then attached (after removing thev stopper from its lower end) to the lower part of the sample collecting train. t

The upper assembly I9 forming the top of the sample collecting train is then put in place. As best shown in Figure2, this includes a T 20, a short nipple 2| and another T 22. The upper end of nipple 2l carries a rubber `stopper 23 in which is a glass tube 24, one end of which is drawn to capillary dimensions. The outlet of T 20 is stoppered at'this stage of the procedure with a'blank stopper and the outlets 26 of T 22 are preferably permanently covered "with wire gauzel21 to prevent intrusion of soil particles. After the upper part of the sample collecting train is assembled, the stopper is removed from the top of the uppermost sorbent section I8 and the upper 'assembly I9 is screwed in place. cell 28, contained in waterproof case Z9, with waterproof insulated leads passing through rubber stopper 3| to heating coil 32, is then put in place on board I3 or above ground if preferred. Outlet 25 of T 20 is unstoppered and the stopper 3l carrying heating coil 32 is then -inserted as shown. Soil is now filled in to hole II to prevent intrusion of atmospheric air from above the surface of the ground into the sample collecting train. However, T 22 is allowed to project above the surface of the ground or it mayv Turning now to Figures 3 and 4 we see a formv o! apparatus for use in our invention which dif- .fers from that of Figures 1 and 2 in several respects. In Figure 3 the gas entrance section vI5 is extended practically throughout the length of hole I2 and the collecting train extends largely above'the surface of the ground. One of couplings I1 (as shown, the lowermost one) carries a sealing device which may consist of a plate 33 with one or more cylindrical downwardrprojections 34 which serve to seal the hole I2 from the atmospheric air.

' The device of Figures 3 and 4 also differs from that of Figures 1 and 2 in that in place of circulation being induced by an electrical heating coil.' it is induced by a small fan 35 which may be carried by elbow 36 and which is operated by a very small electrical motor 31 which can be run by a battery of dry cells or by a small storage battery 38 with which it is connected by leads 39. This gives somewhat greater and more controllable circulation than does the heating coil of Figures 1 and 2.

be buried a vshort distance below the surface in which ase the soil should not be packed tightly section I5 and out through capillary 24 and T 22. The circulation is maintained at a low rate,

for instance 1 to 10 cubic centimeters of air per 55 minute. The rate selected should in any given instance be as high as possible for practically complete sorption so that the sample collecting operation will not occupy an excessive length of time. However, there is nothing to be gained by passing the gas throughv the device so rapidly that sorption is incomplete and in fact to do so makes the results unreliable.' The ordinary dry cell of the 1.5 volt, 30 ampere-hour type will raisev the temperature of 100 cubic centimeters of air by 5, centigrade for about- 2000 hours so that any reasonable length of sample collecting time can be used. In general this time should be suilicient to pass a` volume of gas equal to at least several times the total volume of hole I2 through Ythe sample collecting train. .Sample collecting times ranging from a few days to a few weeks be used and even shorter times can be used,

particularly with the apparatus shown in Figure 3.

Y Also in place of capillary 24 of Figures 1 and 2, we have provided a needle valve 24 which together with fan 35 serves to control the amount of gas passing through the'sample collecting train. This amount of gas should, of course, be standardized for any given series of samples and the apparatus can readily be calibrated so that for a given sorbent train, fan speed and valve opening the rate of gas circulation will be the desired standard value or the gas actually drawn through the apparatus during the sampling operation can be metered.

In the course of geochemical prospecting operations, the sample collecting process above described is, of course, carried out for each of a. considerable number of survey stations located at suitably spaced points, for instance every onetenth mile along a survey line or over a survey area. The sorbents for hydrocarbons collected at each survey station are transported to a laboratory where the hydrocarbons of interest, notably gaseous hydrocarbons heavier than methane, and also any other substances which may be ofinterest, for instance hydrogen, caribe removed by methods normally including the use of heatA and vacuum. They can then be analyzed by any of a variety of procedures known to the art.

In collecting the samples in a given survey, it is, of course, highly desirable that everything possible be standardized so that the results will ,be strictly comparable and therefore indicative of deep-seated oil and gas deposits underlying the soils from which the 'samples are collected. Thus, for instance, the amount, nature and physical condition ofthe sorbents used, and particularly the sorbent for hydrocarbons, should be carefully standardized. The natureY of the holes and the arrangement of the sample collecting apparatus should be identical and the rate of -conduit for sealing the gas-collecting' zone from the atmosphere when said terminal section is disposed in said zone, at least one of the sections of the conduit near said terminal section containing a solid soil gas purication reagent, one of the other sections, further. removed from said terminal section, containing a sorbent for soil gas, means for separately maintaining the reagent and sorbent in their respective sections, and means disposed in said conduit for inducing positive flow of soil gas from said zone, through said terminal section,V the s olid soil gas purication agent, the sorbent for soil gases and to the atmosphere.

2. Apparatus as claimed in claim 1 in which said flow-inducing means comprises an electrical resistance heated by a battery.

3. Apparatus for sorbing soil gases from a gascollecting zone comprising a conduit made up in a plurality of separable sections, one terminal section of said conduit being adapted for disposition entirely within said zone and to collect the soil gases therefrom, means associated with A the conduit for sealing they gas-collecting zone from the atmosphere when said terminal section is disposed in said zone,at least two of the sections near said terminal section containing respectively a sorbent for acidic gases and a desiccant, one of the other sections, furtherremoved.

lfrom said terminal section, containing a sorbent for soil gases, means for separately maintaining the sorbents and desiccant in their respective sections, and means disposed in said conduit for inducing a positive flow of soil gas from said zone, through said terminal section, the sorbent for acidic gases and the desiccant, the sorbent for soil gases and to the atmosphere.

4. Apparatus for sorbing soil gases from a gascollecting zone comprising a conduit made up in' a 4plurality of separable sections. one terminal section of said conduit being adapted for disposition entirely within said zone and to collect the-soil gases therefrom, means associated with the, conduit for sealing the gas-collecting zone from the atmosphere when said terminal section is disposed in said zone, at least two of the sections near said terminal section containing respectively a sorbent for basic gases and a desiccant, one of the other sections further removed from said terminal section,-containing a sorbent for soil gases, means for separately maintaining the sorbents and desiccant in their respective sections, and means disposed in the said conduit for inducing a positive flow of soil gas from said zone through said terminal section, the sorbent for basic gases and the desiccant, the `sorbent for soil gases and to the atmosphere.

5. Apparatus for sorbing soil gases from a gascollecting zone comprising a conduit made up in aplura1ity of separable sections, one terminalv section of said conduit being adapted for disrposition entirely within said zone and to collect for hydrocarbon soil gases, said first-mentioned desiccant vbeing nearest said terminal section and the sorbent for hydrocarbon soil gases being furfor separately maintaining the sorbents and desicther removed lfrom said terminal section, means cants in their respective sections, and means disposed in said conduit for inducing a positive flow of soil gas from said zone, through said terminal section and thence in order through the firstmentioned desiccant, the selective sorbent for basic gases, the basic sorbent for acidic gases, the

.second-mentioned desiccant, the sorbent for hydrocarbon soil gases and thence to the atmosphere.

DONALDl S. REARDEN. EARL G. BREWSTER. R. THOMAS sANDERsoN. 

